1. Field of the Invention
The present invention relates generally to optical imaging systems, and more particularly to an improved wavefront sensor for providing an accurate measurement of the wavefront errors in adaptive optics imaging systems.
2. Brief Description of the Prior Art
Many imaging systems are limited in their performance by the presence of optical wavefront errors. Adaptive optics are capable of removing these wavefront errors but only if an accurate measurement of the wavefront is available. Therefore, a wavefront sensor must be incorporated into the imaging system.
Hartmann wavefront sensors and shearing interferometers have been used in the past for estimating wavefront errors in adaptive optics systems.
Hartmann wavefront sensors, however, are limited in that they can only make measurements from point sources, such as a star or a laser beacon. Thus, such wavefront sensors require the use of locally generated reference beams in order to measure such quantities as image intensity or wavefront phase or tilt. The use of additional reference beams is undesirable as they add to the overall complexity of the wavefront sensor and can introduce additional sources of scattered light which can have an adverse affect on the measurements of interests.
Of particular interest for imaging systems is a wavefront sensor which uses photons collected from the object scene to provide information on the optical aberrations of the imaging system. A shearing interferometer is an example of such a wavefront sensor but a shearing interferometer also is optically very complex as it requires reimaging optics for measurement of the wavefront at a pupil.
Phase retrieval is a known technique whereby the optical-system aberrations are derived directly from image intensity-data. The wavefront sensors used in the phase retrieval technique are therefore very simple. However, phase-retrieval algorithms which use only a single image of an object scene require some knowledge of the object-scene content such as the location of isolated point sources within the imaged field of view. This limits their usefulness to particular applications for which appropriate knowledge of the scene content is available.
Phase diversity is an extension of the phase retrieval concept whereby two images, one containing a known aberration, are compared to determine the optical-system aberrations. Phase diversity algorithms that are independent of the object-scene content can be defined, making them useful for a broad range of adaptive optics applications. The phase diversity concept is described in a paper by R. A. Gonsalves entitled "Phase retrieval and diversity in adaptive optics," Opt. Eng. 21 829-832 (1982).
It would be desirable to develop an optically simple wavefront sensor which is effective to receive a single optical input beam and provide two images of an object scene as output, wherein one of the images has an additional known aberration. Such a wavefront sensor could then be used in combination with phase diversity techniques for estimating the wavefront errors of an optical system.
It would further be desirable to develop such a phase diversity wavefront sensor which could be easily implemented in conventional staring and scanning type imaging systems with little or no modifications to the existing focal plane architecture of such staring and scanning imaging systems.